1. Technical Field
The present invention relates to a switch circuit.
2. Related Art
Active elements employed in a switch circuit that operates under a microwave band or a millimeter-wave band (millimeter-wave band covers a range of 30 GHz to 300 GHz) include a PIN diode and a field effect transistor (hereinafter, FET), each of which has its characteristics. In particular, to reduce insertion loss and improve isolation performance it is essential to reduce ON resistance and OFF capacitance of the active element, for which the PIN diode is more superior. In many of the millimeter-wave switch circuits of 30 GHz or higher especially, the PIN diode is employed for reducing the resistance and the capacitance. The PIN diode is, however, inferior to the FET in the following aspects. The PIN diode has poor compatibility with a heterojunction transistor process to constitute most of millimeter-wave monolithic integrated circuit (MMIC), and consumes a larger power under a low resistance.
The FET can be handled as a two-terminal device when simplified, so as to be utilized as an ON resistance Ron between the source and the drain when the channel is open, and employed as an OFF capacitance Coff between the source and the drain when pinched off, in the circuit.
From the viewpoint of circuit configuration, various types have been developed and commercialized, such as a resonance type, a non-resonance type and distributed constant type (traveling wave type). The resonance type is less advantageous in achieving a broadband characteristic, because of depending on resonance. The non-resonance type is, for example, built with a series-shunt configuration of the active element (FIG. 9A). This is quite advantageous for a broadband operation because of not utilizing the resonance, however unable to operate under high frequency (no more than 60 GHz or so) when employed in a Single Pole n-Throw (hereinafter, SPnT) switch, for the following reason.
As described in the non-patent document 1 (H. Mizutani et al., IEEE Trans. MTT, Vol. 46, No. 11, pp. 1597-1603, November 1998), the OFF branch of a SPST switch can be equivalent to a series capacitance-shunt resistance configuration (FIG. 9B). An increase in frequency leads to reduced impedance of the series capacitance, and hence isolation characteristic is degraded (FIG. 10). In contrast, the ON branch can be equivalent to a series resistance-shunt capacitance configuration (FIG. 9C), and accordingly the insertion loss increases with the increase in frequency (FIG. 10). Consequently, the ON/OFF ratio, which is an important factor of the switch, is degraded. As a result, if the ON/OFF ratio of 20 dB or more is required in the foregoing configuration, the upper limit of the frequency is around 60 GHz (FIG. 10).
Meanwhile, the patent documents 1 and 2 (Japanese Laid-open patent publications No. 2910681 and No. 3099880) disclose a traveling wave type SPST switch that includes a distributed constant FET, which achieves low power consumption and high compatibility between the heterojunction FET process. The non-patent document 2 (H. Mizutani et al., IEEE Trans. MTT, Vol. 48, No. 5, pp. 840-845, May 2000) also describes the operation of such switch in details. The distributed constant FET refers to, as shown in FIG. 11, a one gate finger structure FET including a pair of ohmic electrodes (source electrode and drain electrode) disposed across the gate electrode, in which the gate finger length l including the ohmic electrode is 1/16 or longer of the propagation wavelength.
Referring to FIG. 12, an equivalent circuit of the distributed constant FET can be expressed as a circuit including an infinite number of FETs of a minute length connected to one another via the gate, and a transmission line constituted of the drain electrode of each FET, thus constituting the distributed constant FET of a finite length. The circuit of FIG. 12 can also be expressed as FIG. 13, based on a lumped constant element. In an ON state the distributed constant FET is pinched off, and hence the shunt conductance G equals 0 S. Accordingly, the FET operates in the equivalent circuit associated with a lossless transmission line, thereby achieving a low insertion loss characteristic in the broadband (FIG. 14A). In an OFF state in contrast, the distributed constant FET is in an open channel state, so as to act as the equivalent circuit associated with the transmission line that incurs a loss primarily originating from the shunt conductance G, as shown in FIG. 13. Because of an increase in impedance caused by the series inductance LTL, the broadband characteristic that the isolation monotonously increases with the frequency is achieved (FIG. 14B).
Thus, the traveling wave type switch including the distributed constant FET is quite useful in achieving the broadband characteristic. A report on the SPnT switch including the distributed constant FET, however, can only be found in a circuit including a coplanar waveguide reviewed hereunder, and no report is available yet regarding a circuit including a microstrip line. Accordingly, development of a traveling wave type SPnT switch including the distributed constant FET with a microstrip line has been eagerly sought for.
FIG. 15 is a circuit diagram of the SPDT switch according to the patent document 3 (Japanese Laid-open patent publication No. H09-162602). A diverging point a-end is connected to a grounded PIN diode 103 via a transmission line 101 of an electrical length of θ, and θ is 90° (=Λ/4, where Λ is a propagation wavelength), according to the disclosure. This circuit is described to operate as follows. When the PIN diode 103 is biased forward, the circuit can be considered as merely being equivalent to a resistance Rs, and when the PIN diode 103 is biased reversely, the circuit can be considered as merely being a capacitance Cj.
Generally, the ground point (short point) is converted to be open when its impedance is seen through the transmission line of Λ/4 in length. Accordingly, the resistance Rs is quite small when the PIN diode 103 is biased forward, and hence in the SPDT switch shown in FIG. 15, the impedance is converted to be substantially open at the a-end when seen from the substantially short point through the transmission line of Λ/4 in length. Under such state a microwave signal is substantially totally reflected by the a-end toward the circuit on the side of the transmission line 101, and transmitted to the side of the transmission line 102 with low loss. In contrast, when the PIN diode 103 is biased reversely, the shunt capacitance Cj acts as a part of a shunt capacitance constituting an equivalent circuit of the transmission line 101, so that a current runs through both of the transmission lines 101, 102. Accordingly, the microwave signal has its input power E split into ½ each at the a-end to be supplied to the transmission lines 101, 102 respectively, and then to a load connected to a b-end and c-end. In an ordinary SPDT switch, unlike the example of FIG. 15, the grounded diode is also connected to the c-end on the side of the transmission line 102 as on the side of the transmission line 101, so as to complementarily switch the bias of those transmission lines, thereby switching the propagation path of the microwave signal between the transmission line 101 and the transmission line 102. Such circuit is, despite being popularly utilized, difficult to achieve a high isolation characteristic in the broadband.
FIG. 16 is a circuit diagram of a SPDT switch according to the patent document 4 (Japanese Laid-open patent publication No. 2002-33602). The SPDT switch includes the distributed constant FET. Between ground lines between coplanar waveguides 118a, 118b and between coplanar waveguides 128a, 128b, inserted between a diverging point A and distributed constant FETs 111, 121, FETs 112, 113 and FETs 122, 123 are respectively inserted in series.
In the circuit thus configured, for example, pinching off the FETs 112, 113 on the OFF branch side disconnects the ground line on the OFF branch side, which allows blocking leakage of the signal power to the OFF branch, thereby improving the signal power transmission characteristic to the ON branch side, resulting in minimized insertion loss of the SPDT switch as a whole.
FIG. 17 is a circuit diagram of a traveling wave type SPDT switch according to the non-patent document 3 (K-Y. Lin et al., IEEE Trans. MTT, Vol. 52, No. 8, pp. 1798-1808, August 2004). This SPDT switch operates based on a similar principle to that of the traveling wave type switch including the distributed constant FET according to the patent documents 1, 2 and non-patent document 2. However, while the distributed constant FET that can be expressed as a complete distributed constant circuit is employed to constitute the traveling wave type switch in those cited documents, the SPDT switch according to the non-patent document 3 is different in that three basic cells including a combination of a separated FET and a transmission line are connected in series, thus simulatively constituting a traveling wave type switch.
As shown in FIG. 17, a diverging point is connected to the FET constituting the traveling wave type switch, via a transmission line having a length of Λ/4 of the propagation wavelength. Although the non-patent document 3 states that the transmission line from the diverging point to the FET is actually shorter than Λ/4 because the impedance Za1 of the traveling wave type SPST switch cell is not entirely substantial, the document includes no reference that enables determining a specific length of the transmission line. In addition, the non-patent document 4 (J. Kim et al., IEEE Microwave and Wireless Components letters, Vol. 13, No. 12, December 2003) also discloses a switch circuit based on a similar traveling wave type switch technique to that of the non-patent document 3. According to the non-patent document 4, a diverging point is connected to the distributed SPST switch, via a transmission line having a length of Λ/4 of 77 GHz.
As reviewed above, in the conventional SPDT switches the diverging point is connected to the FET or the diode, via the transmission line having a length of Λ/4 or shorter (though a specific length is not disclosed), or via the coplanar waveguide including the FETs inserted in series in the ground line.